/Engine/Plugins/Runtime/nDisplay/Source/SharedMemoryMedia/Public/SharedMemoryMediaSource.h
Mode of operation when receiving data. Framelocked - Matches source and local frame numbers. Always use this mode in nDisplay. Genlocked - It doesn't match frame numbers, but it also doesn't skip frames, so will hold back the sender if it is faster than the receiver. Freerun - It always grabs the latest frame. It may skip frames if they arrive too fast.
enum ESharedMemoryMediaSourceMode { Framelocked = 0, Genlocked, Freerun, }